1. Field of the Invention
The present invention relates to photographic exposure control apparatus and, more particularly, to exposure control apparatus for automatically controlling the exposure interval of a fixed-focus camera in cooperation with a flash unit providing a fixed quantity of flash illumination.
2. Description Relative to the Prior Art
A number of prior art cameras adapted for use with a fixed output flash device, such as a flash bulb or a non-quenching electronic flash unit, and film of a particular light sensitivity ("speed") operate, in a flash mode, with a selected combination of shutter time and exposure aperture that provides a nominally "correct" exposure at one intermediate, predetermined subject distance. The exposure latitude of the film is relied upon to produce acceptable photographs at other subject distances. For example, the Kodak Instamatic 300 camera, in a flash mode has an f/8 exposure aperture and a 1/40th of a second shutter speed. With this combination of aperture and shutter speed, and with a flashcube, which has a guide number of about 64, and ASA 64 film, photographs are nominally correctly exposed at 8 feet, one stop overexposed at 5.6 feet, two stops overexposed at 4 feet, one stop underexposed at 11.3 feet and 2 stops underexposed at 16 feet. Thus, to produce acceptable photographs at all useful flash ranges (4 to 16 feet), the Instamatic 300 camera requires the use of film having an exposure latitude of at least .+-.two stops. This exposure latitude is widely available in films adapted for use with conventional photographic cameras. For example, acceptable photographs can be produced from conventional print films with up to 3 stops underexposure and up to 2 stops of overexposure.
The so-called "instant" films used in self-developing cameras, however, typically have an exposure latitude of approximately .+-.one-half stop. To provide acceptable flash photographs with film of such narrow exposure latitude, designers provide in self-processing cameras: (1) a quenchable flash unit and/or (2) an automatic exposure control system responsive to both ambient and flash light. The disadvantage of each of these approaches is that they add to the complexity and cost of the camera.